The new conclusions challenge the popular conception that women cheat because they are generally unhappy with their relationships, said study lead author Eric Anderson, a professor of sport, masculinity and sexuality at the University of Winchester in England.

"People will often say that infidelity is a sign of a deeper relationship trouble," Anderson said. But that storyline largely comes from the therapist's couch, after a woman has gotten caught cheating, he added.

"When you get caught doing anything that is highly stigmatized, you make excuses. The excuse becomes, 'Well my husband doesn't treat me well, oh there's a problem in the relationship,'" said Anderson, who is chief science officer at AshleyMadison.com. [6 Scientific Tips for a Successful Marriage]

The findings will be presented on August 18 at the American Sociological Association's 109th annual meeting.

Causes of cheating

Though monogamy is the norm in modern society, adultery is fairly common, with up to one-third of men and one-quarter of women in the U.S. having at least one affair outside of their primary relationship at some point in their lives, according to a 2011 study in the journal Archives of Sexual Behavior.

Still, someone's justifications after the fact may be very different from their initial reasons for straying, Anderson said. Women lose interest in sex with their partners the longer they are in a relationship, according to a 2012 study in the Journal of Sex & Marital Therapy. So it may be that, just like men, female cheaters are seeking an extra sexual frisson, he said.

Seeking affair

Anderson and his colleagues gained access to the profiles and messages sent by 100 married, heterosexual women, who listed their age as between 35 and 45. All told, that included about 42,000 lines of text, Anderson said. Most of the women exchanged a few messages with potential partners before taking their interaction offline or ending conversation altogether, he said.

"They didn't know I was reading their conversations," which enabled Anderson to study their self-professed motivations, without them worrying about being judged, Anderson said. (When they sign up for AshleyMadison, users agree to terms of service that include the possibility that their profile information and usage of the site may be studied for research purposes, Anderson said.)

About two-thirds of the women said they were seeking more romantic passion, which always involved sex. And none of the women wanted to leave their spouses, with many even talking up their husbands, the researchers found.

Though the findings are limited to just one dating website — and one catering to people seeking to cheat — they suggest that many women who are unfaithful are not dissatisfied overall with their partners, Anderson said.

Emotional or rational?

Most of the women wanted just one affair partner and sought at least some emotional connection, Anderson said.

"They needed a bit of a relationship to have an affair, whereas men are more willing to go off and just have sex with whoever," Anderson said.

It's possible that women need more emotional connection to enjoy sex, or that they are simply being rational, so they choose to have a single, more involved, affair, he said.